In order to modify coating layers of natural or synthetic resins, attempts are made to add various modifiers to the resin matrix or to impart a porous structure to the coating layer.
A typical method of forming a porous coating layer comprises the steps of forming a resinous solution by dissolving polyurethane resin with a water soluble solvent, coating the substrate with the resin solution, immersing the substrate in water to solidify and remove the solvent with water.
Porous coating layers can also be obtained by blending polyurethane resin with water and solvent to form a W/O type emulsion, coating the emulsion on a substrate and evaporating the water and the solvent by drying.
These methods are defective in that they involve special equipment and processes, that the porous structure in the coating layer is easily breakable, that the pores in the porous structure do not exhibit sufficient permeability, or that the types of resin that can be used are limited.
In order to obtain porous coating layers of this type, it has been attempted to blend the resin with particles that can be eluted, such as particles of sodium chloride, calcium chloride, sodium carbonate and sugar, and to subsequently remove the particles using water.
Use of water soluble particles such as of sodium chloride enables manufacture of porous coating layers such as films, laminates or sheets of paint or coating using the conventional methods and facilities for producing paints, coatings, etc. in general. It also enables manufacture of resin films, sheets, and various other resin moldings using the conventional molding methods and facilities.
Any and all types of resin that can be used in paints, coatings, etc. can be used to obtain porous coating layers of this type, and the resultant coating layers exhibit strength that is comparable to other coatings in general.
Further, the diameter and the number of pores in the porous coating layers can be suitably controlled by controlling the particle size and the amount of particles to be blended with the resin matrix. This in turn facilitates production of coating layers with desired permeability.
However, porous coating layers obtained by blending resins with particles of sodium chloride, calcium carbonate, calcium chloride or sugar and by eluting the particles with water do not exhibit sufficient permeability nor are they advantageous in terms of modification of the resin matrix.
It is assumed that the insufficient permeability is attributable to the fact that the "pores" in the coating layer are not fully expanded but are present as mere voids after elution of particles.
Based on the above assumption, the present inventors have made an attempt to obtain porous coating layers with modified properties by using gelatin which contain as the principal component proteins having excellent affinity with resins.
An advantage derived from the use of gelatin in combination with natural or synthetic resin to obtain coating layers lies in the moisture absorbing and permeating properties of the gelatin contained therein, which impart to the coating layer excellent moisture absorption and excellent feel without stickiness. Resin coating layers blended with gelatin are also excellent in contact resistance at the surface and are therefore high in adhesion. Coating layers blended with gelatin are further superior in weatherability, particularly cold resistance and are static-free.
By eluting gelatin particles blended in the resin coating layers with cold or warm water, porous coating layers with excellent moisture and air permeability can be obtained.
Elution of gelatin particles blended in the resin coating layers with cold or warm water results in "pores" that are fully expanded; more particularly, even if the pores at the surface are small in diameter, they are sufficiently expanded inside and have larger diameter, imparting to the coating layers high permeability for their resistance against water pressure.
It is noted that very small dents are formed on the surface of expanded pores formed inside the coating layer which, together with the pore openings at the surface, contribute to giving the surface of the coating layer soft texture and good feel without stickiness. These dents also cause irregular reflection of the light incident on the surface.
However, gelatin powders that are commercially available are irregular and often too large in the particle size. When such commercial gelatin powders are used as they are, the pores in the resultant coating layers become too large, and the water pressure resistance as against the required permeability becomes insufficient. Use of gelatin powder with large particle size also results in thick layer of coatings, and coating layers with required air and moisture permeability cannot be obtained.
Repeated experiments by the inventors suggest that gelatin particles to be blended in the coating layers are preferably very fine and are within a given range of particle size.
The inventors made an attempt for mass production of very fine gelatin particles using commercial gelatins and a jet mill.
Pulverization of gelatin in a jet mill enables concurrent classification of powder, and is therefore suitable for obtaining powder with particle size distributed within a given range.
However, use of jet mill for pulverization of gelatin is defective in that pulverized particles tend to become fused or coagulated, making pulverization itself impossible or causing the pulverized particles to adhere to the inner surface of the mill.
The inventors then tried to pulverize gelatin powder from which moisture content was removed as much as possible in a mill wherein the humidity was kept as low as possible. These improvements in the pulverization process made it possible to pulverize and classify gelatin powder into very fine particles within a given range of particle size.
The improved pulverization process using jet mill is still defective in that the yield of pulverization is too small to be employed in practical application. The improved process is further defective in that the gelatin particles tend to become denatured because of the prolonged process and because moisture is further removed from gelatin during pulverization, making elution with warm or cold water impossible.
Further attempt was made by the inventors to use wet ball mill, in which gelatin powder is charged together with organic solvent such as dimethylformamide and wherein the atmosphere is kept dry.
Efficient pulverization was possible in the wet ball mill, with gelatin particles not becoming coagulated or adhered to the inner wall of the pulverizer when the water content of gelatin powder during pulverization was kept at a low level.
Pulverization in the wet ball mill was still defective in that the particle sizes after pulverization were widely varied, and those as large as the starting material at the time of charging and those having very small particle size of 1.5 .mu.m or smaller were both present at certain ratios.
Although the mean particle size can be arbitrarily reduced by suitably adjusting the pulverization conditions, individual particles would vary widely in size to thereby make the particles not suitable for practical use.
Pulverization in the wet ball mill for an extended period of time would produce particles with smaller sizes, but this prolonged pulverization would also produce "over-pulverized" particles as well.
The over-pulverized particles, particularly those having the particle size of 1 .mu.m or smaller would fail to exhibit properties that are unique to gelatin when they are blended with resin, or cannot be eluted from the resultant coating layer with water. Gelatin powder pulverized in the wet ball mill containing a large amount of over-pulverized particles is not suitable for practical use.
Upon repeated experiments, the inventors have succeeded in developing particles of gelatins and amino acids that are efficiently pulverized to particle sizes in a given range and that can be eluted with cold or warm water to give the coating layers various functions imparted by the gelatin powder blended with resins mentioned above.